Method and apparatus for performing beam searching in a radio communication system

ABSTRACT

A radio communication system includes a base station having a directional antenna for generating a plurality of beams. A first set of the beams is used to receive signals which are decoded at the base station. A second set of the beams is used for interrogating a cell to identify beams which should be added to the first set of beams (for example, in response to mobile terminals entering the cell). In one embodiment, the second set of beams comprises a plurality of searcher beams produced by a fixed-beam phased array antenna. In a second embodiment, the second set of beams comprises a single searcher beam which is scanned through the cell by an adaptive phased array antenna. The radio communication system can be used to communicate with indoor mobile terminals through a plurality of radio heads, and also can be used to communicate with orbiting satellites.

This application is a division of Ser. No. 08/971,341 filed Nov. 17,1997 now U.S. Pat. No. 6,694,154.

BACKGROUND

The present invention pertains to a system and method for efficientlycancelling interference in a radio communication system using adirectional antenna and one or more search beams.

FIG. 1 illustrates a conventional cellular radio communication system100. The radio communication system 100 includes a plurality of radiobase stations 170 a-n connected to a plurality of corresponding antennas130 a-n. The radio base stations 170 a-n in conjunction with theantennas 130 a-n communicate with a plurality of mobile terminals (e.g.terminals 120 a, 120 b and 120 m) within a plurality of cells 110 a-n.Communication from a base station to a mobile terminal is referred to asthe downlink, whereas communication from a mobile terminal to the basestation is referred to as the uplink.

The base stations are connected to a mobile telephone switching office(MSC) 150. Among other tasks, the MSC coordinates the activities of thebase stations, such as during the handoff of a mobile terminal from onecell to another. The MSC, in turn, can be connected to a public switchedtelephone network 160, which services various communication devices 180a, 180 b and 180 c.

A common problem that occurs in a cellular radio communication system isthe loss of information in the uplink and downlink signals as a resultof multi-path fading, which results when the transmitted signal travelsalong several paths between the base station and the intended receiver.When the path lengths between the base station and the mobile terminalare relatively small, the multiple signal images arrive at almost thesame time. The images add either constructively or destructively, givingrise to fading, which typically has a Rayleigh distribution. When thepath lengths are relatively large, the transmission medium is consideredtime dispersive, and the added images can be viewed as echoes of thetransmitted signal, giving rise to intersymbol interference (ISI).

Fading can be mitigated by using multiple receive antennas and employingsome form of diversity combining, such as selective combing, equal gaincombining, or maximal-ratio combining. Diversity takes advantage of thefact that the fading on the different antennas is not the same, so thatwhen one antenna has a faded signal, chances are the other antenna doesnot. ISI from multi-path time dispersion can be mitigated by some formof equalization, such as linear equalization, decision feedbackequalization, or maximum likelihood sequence estimation (MLSE).

Interference can also degrade the signals transmitted between a basestation and mobile terminals. For instance, a desired communicationchannel between a base station and a mobile terminal in a given cell canbe degraded by the transmissions of other mobile terminals within thegiven cell or within neighboring cells. Other base stations orRF-propagating entities operating in the same frequency band can alsocreate interference (through “co-channel” or “adjacent channel”interference).

Frequency re-use can be used to mitigate interference by locatinginterfering cells as far from each other as possible. Power control canalso be used to reduce the interference by ensuring that transmitterscommunicate at minimal effective levels of power. Such power controltechniques are especially prevalent in code-division multiple accesssystems, due to the reception of information in a single communicationchannel at each base station.

Interference can be reduced still further by using a plurality ofdirectional antennas to communicate with mobile terminals within a cell.The directional antennas (also known as “sector antennas”) transmit andreceive energy within a limited geographic region, and thereby reducethe interference experienced by those radio units outside suchgeographic region. Typically, radio communication cells are partitionedinto three 120° sectors serviced by three sector antennas, or six 60°sectors serviced by six sector antennas. Even smaller antenna sectorscan be achieved using a fixed-beam phased array antenna, which transmitsand receives signals using a plurality of relatively narrow beams. FIG.2, for instance, illustrates such an exemplary radio communicationsystem 200 including a radio base station 220 employing a fixed-beamphased array (not shown). The phased array generates a plurality offixed narrow beams (B₁, B₂, B₃, B₄, etc.) which radially extend from thebase station 220. Preferably, the beams overlap to create a contiguouscoverage area to service a radio communication cell. Although not shown,the phased array can actually consist of three phased array sectorantennas, each of which communicates with a 120° swath extending fromthe base station 220.

FIG. 2 shows a mobile terminal 210 located within the coverage of one ofthe beams, B₁. Communication proceeds between the base station 220 andthis mobile terminal 210 using the beam B₁, or perhaps, in addition, oneor more adjacent beams. The reader will appreciate that modern radiocommunication environments typically include many more mobile terminalswithin cells. Nevertheless, even when there are plural mobile terminalswithin a cell, a subset of the beams may not include any mobile terminalstations within their coverage. Hence, in conventional fixed-beam phasedarray systems, these beams remain essentially idle until a mobileterminal enters their assigned geographic region. Such idle beamspropagate needless energy into the cell, and thus can contribute to thenet interference experienced by radio units within the cell as well asother cells (particularly neighboring cells). These beams also add tothe processing and power load imposed on the base station 220.

These concerns are partly ameliorated though the use of a variation ofthe above-discussed system, referred to as “adaptive” phased arrays.Such arrays allow for the selective transmission and reception ofsignals in a particular direction. For instance, as shown in FIG. 3, anarray 300 can be used to receive a signal transmitted at an angle θ(with respect to the normal of the array) from a target mobile terminal380, and can simultaneously cancel the unwanted signals transmitted byanother mobile terminal 370. This is accomplished by selecting weights(w₁, w₂, . . . w_(n)) applied to each signal path (r₁, r₂, . . . r₃)from the phase array antenna 300 so as to increase the sensitivity ofthe array in certain angular directions and reduce the sensitivity ofthe array in other directions (such as by steering a null toward aninterference source). The desired weighting is selected by iterativelychanging the weights through a feedback loop comprising beamforming unit340, summer 330 and controller 320. The feedback loop functions tomaximize signal-to-interference ratio at the output “x” of thebeamforning unit. Application of an adaptive phased array antenna to theradio communication system shown in FIG. 1 would result in thegeneration of a single beam (or small subset of beams) generallyoriented in the direction of the single mobile terminal 210. Such asystem offers a substantial reduction in interference. For example, asdisclosed in “Applications of CDMA in Wireless/Personal Communications”by Garg et al., Prentice Hall, 1997, an idealized eight-beam antennacould provide a threefold increase in network capacity when comparedwith existing schemes such as cell splitting (pp. 332-334). Interestedreaders are referred to the following documents for further detailsregarding adaptive phased arrays as well as information regardingadaptive diversity arrays: “Adaptive Arrays and MLSE Equalization” by G.E. Bottomley et al., Proc. VTC '95, Chicago, Ill., July 1995, pp. 50-54;“Signal Acquisition and Tracking with Adaptive Arrays in the DigitalMobile Radio System IS-54 with Flat Fading” by J. H. Winters, IEEETransactions on Vehicular Technology, Vol. 42, No. 4, Nov. 1993;“Adaptive Array Methods for Mobile Communication” by S. Simanapalli,Proc. 44th IEEE Veh. Technol. Conf., Stockholm, Sweden, Jun. 7-10, 1994,pp. 1503-1506; and published patent application No. WO 94/09568 to P. H.Swett et al., published 1994.

The presence and location of mobile terminals in both the fixed andadaptive beamforming cellular radio communication systems can bedetermined by measuring the signal strength in the uplink direction oneach beam. The beam direction yielding the strongest received signalwould indicate the probable location of the desired mobile. Thistechnique, however, is not fully satisfactory. Often, for instance, dueto multi-path fading, the beam yielding the strongest signal may notprecisely correspond to the direction of the mobile user. Even if thestrongest beam does correspond to the direction of the mobile user, thepresence of multi-path fading and interference on other beams maydegrade the quality of communication between the base station and themobile terminal using the strongest beam. Furthermore, successivelyexamining each beam generated by the phased array to locate a mobileuser requires a significant amount of processing overhead. This overheadcan reduce the response time of the base station.

It is therefore an exemplary objective of the present invention toprovide a method and system for conducting communication between tworadio units which does not suffer from the above-described drawbacks.

SUMMARY OF THE INVENTION

According to a first exemplary aspect of the present invention, theabove objective is achieved through a base station using a fixed-beamphased array antenna which employs a first set of beams and associatedhardware for conducting communication with a set of mobile terminalswithin a radio communication cell, and employs a second set of beams andassociated hardware for searching the radio communication cell for thepresence of candidate beams which should be added to the first set ofbeams. In the following discussion the beams in the first set arereferred to as “decoding beams”, while beams in the second set aredenoted “searcher beams”.

According to a second exemplary aspect of the present invention, subsetsof the decoding beams are processed by an equalizer, and are preferablyprocessed by the interference-rejection-combining receiver disclosed incommonly assigned U.S. application Ser. No. 07/284,775, filed on Feb. 8,1994. This receiver combines signals received from each subset ofdecoder beams and separates the wanted signals from the unwanted(interfering) signals.

According to a third exemplary aspect of the present invention, the basestation determines the “membership” of each subset of decoder beams bysuccessively examining each beam within the searcher set of beams. Thosesearcher beams (or combination of searcher beams) which meet prescribedcriteria are selected and allocated to the task of processing a callfrom a mobile terminal. The beam is “allocated” in the sense that itsassociated hardware (e.g. comprising filters, downconverters, etc.) isallocated to the task of processing the call.

According to a fourth exemplary aspect of the present invention, thesearcher beams and their associated hardware are used to determine thepresence of new mobile terminals within a cell, including thoseterminals which have entered the cell from a neighboring cell, and thoseterminals which have initiated calls within the cell. The searcher beamsand associated hardware are also used to determine the departure ofterminals within a cell, including those terminals which have physicallyleft the cell and those terminals which have simply terminated callswithin the cell.

According to a fifth exemplary aspect of the present invention, thereceiver/equalizer also interrogates the allocated decoder beams todetermine whether these beams continue to possess signal characteristicswhich warrant their membership in the decoder set of beams. If a decoderbeam no longer meets the prescribed criteria, it is returned to thesearcher pool of beams. Thus, the allocation of beams (and associatedhardware) to the decoder beam set and the searcher beam set is a dynamicprocess which takes into account all activity within the cell andoutside the cell which affects the interference profile within the cell.According to one exemplary criterion, searcher beams are converted intodecoder beams when they contain signal strength and/or signal qualitycharacteristics above a prescribed threshold.

According to a sixth exemplary aspect of the invention, instead of afixed beamforming phased array antenna, the base station can employ anadaptive phased array antenna. In this embodiment, a single searcherbeam can be used to interrogate the cell to recruit candidates forinclusion in the decoder set of beams and to determine out-of-datemembers in the decoder set of beams. By appropriate weighting of thephased array, the base station steers the single searcher beam over aprescribed swath of geographic coverage. In alternative exemplaryembodiments, more than one searcher beam can be employed.

According to a seventh exemplary aspect of the invention, theabove-described cellular techniques can be employed in the indoorcellular environment. In this case, the radio heads are divided into afirst set of decoder radio heads which are allocated to the task ofprocessing calls, and a second set of radio heads which are allocated tothe task of ensuring that the decoder set of radio heads remains optimalor near-optimal. Again, the signals provided by the decoder set of radioheads are processed using a receiver, preferably using theinterference-rejection-combining receiver mentioned above.

According to an eighth exemplary aspect of the invention, theabove-described cellular techniques can be used by a base station tolocate one or more orbiting satellites by employing a decoder set ofbeams which are assigned for communicating with one or more satellitesand a second searcher set of beams for canvassing a sector of space toensure that the decoder beams remain optimal or near-optimal byrecruiting searcher beams which meet prescribed criteria for inclusionin the decoder set of beams and rejecting decoder beams which fail tomeet the prescribed criteria.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects, features and advantages of the present invention,as well as other features, will be more readily understood upon readingthe following detailed description in conjunction with the drawings inwhich:

FIG. 1 shows a conventional radio communication system including pluralbase stations and a central switching center;

FIG. 2 shows a conventional base station which uses a phased array witha fixed beamforming processor;

FIG. 3 shows a block diagram of a base station which uses a conventionaladaptive phased array;

FIG. 4 shows a base station which uses a phased array with a fixedbeamforming processor according to exemplary aspects of the presentinvention;

FIG. 5 shows an exemplary block diagram of processing circuitry used bythe base station of FIG. 4;

FIG. 6 shows an exemplary block diagram of aninterference-rejection-combining receiver for use in the base stationcircuitry of FIG. 5;

FIG. 7 shows a base station which uses a phased array with an adaptivebeamforming processor according to exemplary aspects of the presentinvention;

FIG. 8 shows an exemplary block diagram of processing circuitry used bythe base station of FIG. 7;

FIG. 9 shows an indoor radio communication system which uses radio headsto communicate with mobile terminals;

FIG. 10 shows an exemplary radio head for use in the system of FIG. 9;and

FIG. 11 shows a base station which communicates with one or moresatellites using decoder beams selected by a searcher beam or beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular circuits,circuit components, techniques, etc. in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone skilled in the art that the present invention may be practiced inother embodiments that depart from these specific details. In otherinstances, detailed descriptions of well-known methods, devices, andcircuits are omitted so as not to obscure the description of the presentinvention.

The exemplary radio communication systems discussed herein are describedas using the time division multiple access (TDMA) protocol, in whichcommunication between the base station and the mobile terminals isperformed over a number of time slots. However, those skilled in the artwill appreciate that the concepts disclosed herein find use in otherprotocols, including, but not limited to, frequency division multipleaccess (FDMA), code division multiple access (CDMA), or some hybrid ofany of the above protocols.

A. Cellular Radio Communication with Fixed Beamforming

FIGS. 4-6 illustrate a first exemplary embodiment of the presentinvention which entails the use of a base station 440 having adirectional antenna to generate a plurality of narrow beams, whichcentrally radiate from the base station 440. The directional antenna iscontrolled by a fixed beamformer such that the beams are positioned atfixed locations. The number, strength, weighting, and coverage area ofeach beam can be selected so that the beams collectively provide therequired coverage for a particular application. One or more phasedarrays can be employed to achieve the desired coverage, or other typesof directional antennas can be used instead of a phased array. In theexample of FIG. 1, three separate sector antennas spanning 120° sectorsare used, each of which generates a plurality of narrow beams using aphased array or other type of directional antenna. These sectorboundaries are demarcated by lines 410, 420 and 430.

The base station 440 shown in FIGS. 4-6 projects a first set of beamsreferred to as “decoder beams”. The decoder beams (denoted by the symbol“D”) are used for processing a call from one or more mobile stations.Each of the decoder beams is preferably allocated a “receiver chain” ofprocessing modules (not shown) for processing and conditioning thesignals received from the respective beams, including, for example, anamplifier, downconverter, filter, digital-to-analog converter, etc.Thus, reference to the use the decoder beams to process calls frommobile terminals also implies the allocation of a dedicated receiverchain for processing such calls. In FIG. 1, beams D_(x1), D_(x2) andD_(x3) are used to transmit and receive signals to and from,respectively, at least mobile terminal 450. Beams D_(y1), and D_(y2) areused to transmit and receive signals to and from, respectively, anothermobile terminal 460. Transmission may employ a subset of these beams.

Subsets of the decoding beams are processed by receiver 535 in FIG. 5.Receiver design depends on the modulation used and the performanceneeded. For illustrative purposes, we have assumed MLSE reception ofnarrow-band signals. However, the receiver can be any type. For example,for direct-sequence spread-spectrum systems, the receiver can be acorrelator or Rake receiver. For differentially modulated systems, adifferential detector can be used. Any form of coherent or noncoherentreceiver is possible.

According to preferred embodiments, an interference-rejection-combining(IRC) receiver disclosed in commonly assigned U.S. application Ser. No.07/284,775, filed on Feb. 8, 1994, is used to process the signalsprovided by the decoder beams. This receiver combines signals receivedfrom each subset of decoder beams and separates the wanted signals fromthe unwanted (interfering) signals. This receiver will be discussed infurther detail below. The following commonly assigned U.S. applicationsdisclose IRC concepts and are incorporated in their entireties byreference herein: Ser. No. 08/284,775, filed on Aug. 2, 1994; Ser. No.08/577,337, filed on Dec. 22, 1995; Ser. No. 08/634,719, filed on Apr.19, 1996; and Ser. No. 08/655,930, filed on May 31, 1996.Interference-rejection-combining is primarily discussed herein withreference to the uplink, but can be used to improve the quality of thedownlink transmission as well, as discussed at length in theabove-referenced U.S. application Ser. No. 08/655,930.

FIG. 4 also shows a second set of beams referred to as “searcher beams”,denoted by S₁-S₁₅. To facilitate illustration, in all Figures thedecoder beams are shaded, whereas the searcher beams are not shaded. Thebase station uses the searcher beams to select candidates which shouldbe added to the set of currently active decoder beams by successivelyexamining each beam within the searcher set of beams. Those searcherbeams (or combination of searcher beams) which meet prescribed criteriaare selected and allocated to the task of processing a call from amobile terminal. Once again, the beam is allocated in the sense that itsassociated hardware (e.g. comprising filters, downconverters, etc.) areallocated to the task of processing the call.

The searcher beams and their associated hardware are also used todetermine the presence of new mobile terminals within a cell, includingthose terminals which have entered the cell from a neighboring cell (aswill be the case with terminal 465), and those terminals which haveinitiated calls within the cell. The searcher beams and associatedhardware also provide assistance in determining the removal of terminalsfrom a cell, including those terminals which have physically left thecell and those terminals which have simply terminated calls within thecell.

The receiver 535 also interrogates the allocated decoder beams todetermine whether these beams continue to possess signal characteristicswhich warrant their “membership” in the decoder set of beams. If adecoder beam no longer meets the prescribed criteria, it is returned tothe searcher pool of beams. Thus, the allocation of beams (andassociated hardware) to a decoder beam set and the searcher beam set isa dynamic process which takes into account all activity within the celland outside the cell which affects the interference profile within thecell.

Various criteria can be used to determine whether a searcher beam shouldbe added to the active set of decoder beams. For instance, the basestation 440 can “convert” a searcher beam to a decoder beam when thestrength of the searcher beam exceeds a prescribed value. Theinterference-rejection-combining receiver works best when it receivesthe collection of beams having the strongest interfering signals. Thus,the base station 440 would, in addition to identifying strong signalsattributed to the wanted signal (e.g. from terminal 460 using beamD_(y1)), might also select one or more nearby beams (e.g. beam D_(y2))which contains a strong interfering signal (e.g. attributed to terminal465). Because of multi-path fading effects and other types ofinterference phenomena, the selected subsets of decoder beams may notall be adjacent to one another (as in the case with decoder beamsD_(y1), and D_(y2)). The strength can be gauged by measuring the amountof energy collected from the searcher beam over a certain period oftime, such as over a slot, multiple slots, or a portion of a slot.

In another embodiment, the base station 440 can “convert” a searcherbeam to a decoder beam when the quality of the searcher beam meetscertain criteria. Quality can be gauged, as well known in the art, bymeasuring the correlation of a received signal with a known patternword. For instance, digital transmissions typically include sync wordsat predetermined slots within the transmissions. The quality of thereceived signal can be determined by correlating the received signalwith a sync word. The quality measure can also be used by the receiver535 to identify those beams which should be removed from the active setof decoder beams by identifying those beams having negligible amounts ofwanted signal.

In another embodiment, a hybrid of strength measurements and qualitymeasurements can be used to select the desired set of decoder beams. Forinstance, the strength measure can be used identify the presence ofmobile terminals within an area and select the primary beams forinterference-rejection-combining. The quality measurement can be used toidentify those weaker beams, e.g. attributed to multi-path propagation,which may contain some wanted signal.

In still another embodiment, the introduction of new mobile terminalsinto the cell (or the initiation of new calls within the boundaries of acell) can be determined by detecting the presence of random accesschannel (RACH) transmissions from new mobile terminals. The randomaccess channel RACH is used by the mobiles to request access to thesystem. The RACH logical channel is a unidirectional uplink channel(from the mobile terminal to the base station), and is shared byseparate mobile terminals (one RACH per cell is sufficient in typicalsystems, even during periods of heavy use). Mobile units continuouslymonitor the status of the RACH channel to determine if the channel isbusy or idle. If the RACH channel is idle, a mobile unit desiring accesssends its mobile identification number, along with the desired telephonenumber, on the RACH to the base station. The MSC receives thisinformation from the base station and assigns an idle voice channel tothe mobile station, and transmits the channel identification to themobile terminal through the base station so that the mobile terminal cantune itself to the new channel.

There are typically hardware constraints which restrict the quantity andfrequency of processing which can be performed on the signals receivedby the numerous searcher beams. In view thereof, the base station canlower the “duty cycle” of the processing of the searcher beams. The term“duty cycle” pertains to the amount of time spent by the base station insearching for new mobile terminals. The duty cycle is a function of thefrequency at which each searcher beam is revisited and the amount oftime spent processing each searcher beam upon each visit.

The duty cycle can be lowered by examining only a subset of availablesearcher beams in each slot. The subset of beams processed in each slotcan be varied according to a prescribed schedule, or can be varied in arandom manner. The duty cycle of searcher beam processing can also beadjusted by varying the amount of coding and interleaving used toprocess the signals received from and/or transmitted by the searcherbeams. A longer duty cycle can be achieved by adding to the amount ofcoding and interleaving performed on signals. For instance, coding (suchas repeat coding) tends to add some memory into the transmission interms of spreading bits over a longer period of time. This means thatshorter duty cycles can be achieved by cutting back on the amount ofsuch coding.

According to other exemplary embodiments, the duty cycle at whichsearcher beams are processed can also be optimally selected bydetermining the speed at which mobile terminals move through a cell,which, in turn, can be gauged from the measured Doppler shift of signaltransmissions emanating from mobile terminals within the cell. The speedcan also be known (e.g., it can be assumed that a cell along a highwaywill include mobiles moving at a high rate of speed). More specifically,the frequency at which the searcher beams are interrogated is selectedon the basis of the speed at which the majority of the mobile terminalsare traveling through a cell. For instance, the searcher beams in cellsallocated to fast-moving highway traffic may have to be interrogated ona relatively frequent basis. Searcher beams in cells having a lessdynamic environment can be updated on a less frequent basis.

The measure of signal strength and/or quality can also be used todetermine when to handoff a call from a mobile terminal from one cell toanother. For instance, when the receiver 535 detects that the signalstrength and/or quality for an on-going call decreases below aprescribed threshold, it can notify a central switching center (e.g. asshown in the prior art context in FIG. 1). The switching center willthen coordinate a handoff by informing an adjacent cell to take over thecall. If the switching center is informed of what beam or beams acurrent base station is using to communicate with a mobile terminal, theswitching center can also inform a neighboring base station which beamor beams it should use to resume the call after handoff. This functioncan be implemented, for instance, on the basis of prestored informationwhich indicates which specific beams of a current cell overlap the beamsof a neighboring cell. In the CDMA environment, the mobile station cansimultaneously receive transmissions from plural base stations withinthese overlap regions.

In yet another embodiment, the receiver 535 can be used to track thelocations of mobile terminals as they move within the cell. Based on theprojected paths of the terminals, or trends in interference caused bytheir mutual interaction, the receiver 535 can instruct the base station440 to modify its set of decoder means so as to minimize any degradationin the calls. This function can be performed in an iterative manner suchthat, through trial and error, a more suitable set of decoder beamsoffering improved signal-to-interference ratio is selected, or byreference to some type of knowledge base in which rules are storedregarding proper selection of decoding beam patterns for prescribedinput conditions.

Exemplary circuitry for carrying out the above-described functions willnow be discussed with reference to FIGS. 5 and 6. FIG. 5 shows a circuitincluding an antenna comprising a plurality of antenna elements (e.g.505, 510 and 515). The antenna can comprise a phased array antenna asdiscussed above, or some other directional antenna for producing aplurality of beams. The individual antenna elements (e.g. 505, 510, 515)are connected to a fixed beamformer unit 520, which shapes and steersthe plurality of beams to achieve a desired coverage area, such as toachieve the fixed beam configuration shown in FIG. 1. The beamformer cancomprise any conventional fixed beamformer, such as a Butler matrix. Theconventional beamformer is implemented using analog hardware.Alternatively, digital beamforming can be used. In the presentembodiment, digital beamforming is used in the front-end at someintermediate frequency or appropriately down-converted signal. Thisrequires that the signals remain coherent up to the beamforming stage.Alternatively, the digital beamforming can be performed further awayfrom the front-end of the system. For example, beamforming can beperformed in the baseband signals after filtering and down-conversion.However, such an approach is more complex due to the need to maintaincoherency of the signals over a longer processing path.

Although not shown, other processing units comprising “receiver chains”can be incorporated at the beamformer processing stage or at laterpoints in the transmission path. As well known in the art, such receiverchains can comprise various amplifiers, filters, downconverters,analog-to-digital converters, etc., as will be apparent to those skilledin the art. Each beam can include a respective receiver chain allocatedthereto. However, to reduce hardware costs and processing complexity,the number of receiver chains may be less than the number of antennaelements.

In the receive path, the output of the beamformer 520 comprises aplurality of “M” signals. The M signals 525 are fed into a selector 530which selects a number “N” 550 of the M input signals. These N signals,in turn, are fed to a receiver 535 which collectively analyzes theinformation in the N signals, and, therefrom, extracts the desiredsignals from the unwanted signals. Any type of equalizer, combiner ordetector can be used for the receiver 535. Preferably, aninterference-rejection-combining processor (to be discussed shortlyhereinbelow) is used.

The selector 530 bases its selection of the N signals on the output(“selector control” 560) of a searcher circuit 545. The searcher circuit545 is used to successively examine each of a current pool of searcherbeams. It performs this task by generating and transmitting a selectionsignal 565 to a M-to-N_(s) selector 540 (e.g., which can comprise, inone embodiment, a M-to-1 selector). In response to the selection signal,the M-to-N_(s) selector 540 passes N_(s) (where N_(s) is an integerN_(s)≦M) of the M input signals 525 to the searcher circuit 545 on line570 for analysis therein. The specific analysis performed on eachsearcher beam by the searcher circuit 545 can comprise strength analysisand/or quality analysis, as discussed above, or some other type ofanalysis.

The searcher circuit 545 also receives a searcher control input 555 fromthe receiver 535. This input can, among other control functions,identify those beams which are within the current set of decoder andsearcher beams, and can particularly identify those decoder beams whichare being reallocated to the searcher pool of beams. The receiver 535can also forward raw data to the search circuit 545. However, the bulkof the beam selection analysis is performed by the searcher circuit,since it has more beams available to it than the receiver circuit 535.

The output 560 of the searcher circuit 545 is feed back to the selector530, which commands the selector 530 to select a beam or beams meetingprescribed criteria for decoding using the receiver 535. The output ofthe receiver 535 is a signal S which represents the wanted signal withas much as the unwanted signal removed as possible. The portions of thecircuit shown in FIG. 5 which are responsible for the selection andde-selection of decoder beams can be collectively regarded as an“evaluator circuit”, while the portions of the circuit which perform theactual demodulation and/or interference cancellation functions can bereferred to as the “decoder”. The functions attributed to these circuitscan be implemented using a suitably programmed microprocessor or with acombination of discrete logic devices, as will be apparent to thoseskilled in the art.

The details of the receiver can be found in the above-referencedinterference-rejection-combining (IRC) patent applications, each ofwhich is incorporated herein in its entirety. FIG. 6 shows exemplaryaspects of one embodiment of the IRC receiver. The N received radiosignals 550 generated by the selector 530 are fed into the receiver 535.For the sake of simplicity, only three of the N signals are illustratedin FIG. 6, although it should be noted that, generally, two or moresignals can be included.

The received sample streams 550 (also denoted by r_(a)(n), r_(b)(n) andr_(c)(n) are coupled to a signal pre-processor, or sync, blocks 610, 615and 620, respectively, where the received signal sample streams arecorrelated with known timing/synchronization sequences embedded in thereceived radio signals according to known techniques. Jointsynchronization is also possible. The received signal sample streams arealso coupled to channel tap estimators 625, 630 and 635 to producechannel tap estimates c_(a)(τ), c_(b)(τ) and c_(c)(τ) which are used tomodel the radio transmission channel associated with each antennaelement. Initial channel tap estimates can be obtained from synccorrelation values or least-squares estimation according to knowntechniques. Subsequently, known channel tracking techniques can be usedto update the channel estimates, e.g., using received data and tentativesymbol estimate values generated in the sequence estimation processor640. Joint channel estimation is also possible.

The channel tap estimates c_(a)(τ), c_(b)(τ) and c_(c)(τ) are input tothe branch metric processor 605. The branch metric processor 605 formsbranch metrics which are used by sequence estimation processor 640 todevelop tentative and final estimates of the transmitted informationsymbol sequences. Specifically, hypothesized symbol values are filteredby channel tap estimates from blocks 625, 630 and 635 to producehypothesized received samples for each antenna. The differences betweenthe hypothesized received information and the actual receivedinformation from blocks 610, 615 and 620, referred to as the hypothesiserrors, give an indication of how good a particular hypothesis is. Thesquared magnitude of the hypothesis error is used as a metric toevaluate a particular hypothesis. The metric M_(h)(n) is accumulated fordifferent hypotheses for use in determining which hypotheses are betterusing the sequence estimation algorithm, for example, the Viterbialgorithm.

Also coupled to the branch metric processor 605 is an estimate of theimpairment correlation properties obtained from impairment correlationestimator 600. The estimate of the impairment correlation propertiescomprises information regarding the instantaneous impairment correlationproperties between the antenna elements. The impairment correlationestimator uses impairment process estimates to update and track theestimate of the impairment correlation properties. As distinguished fromconventional techniques, branch metrics formed by processor 605 areimproved by taking into account the correlation between the impairmentassociated with the signals received by the plural antenna elements.This improved branch metric formulation is discussed at great length inthe above-described IRC patent applications, and the interested readeris referred to those disclosures for further information regarding theIRC technique.

According to exemplary aspects of the present invention, the signalstrength of the searcher beams can also be quantified using the desiredchannel tap estimate c. The strength is indicated by the value c^(H)c.The quality of the beam signal can be gauged from c^(H)R⁻¹c, where R isthe impairment autocorrelation matrix generated by the impairmentcorrelation estimator 600.

B. Cellular Radio Communication with Adaptive Beamforming

FIGS. 7 and 8 illustrate a second exemplary embodiment of the presentinvention. In FIGS. 4-6, a fixed beamforming processor 520 is used,whereas in FIGS. 7 and 8, an adaptive beamforming processor 800 is usedin the base station 700. The use of an adaptive beamforming processor800 allows the base station 700 to selectively direct only the requirednumber of beams toward the target terminals (such as terminal 780, whichis serviced by beams D_(z1), D_(z2) and D_(z3)). The collection of beamsis specifically tailored to maximize the signal-to-interference ratio ofthe signals received from the mobile terminal 780.

Furthermore, only one searcher beam S_(scan) is used, or at least asmaller subset of searcher beams are used as compared to the example offixed beamforming. The single searcher beam S_(scan) is steered over arange of orientations. At each orientation, the base station measure thesignal strength and/or signal quality of the searcher beam S_(scan) (orsome other measure), and from this information decides whether thatorientation should be allocated a decoder beam D_(zn). For instance,when the searcher beam reaches the vicinity of the new mobile terminal750, the signal received using the searcher beam may indicate that a newdecoder beam D_(zn) should be established pointing toward the mobileterminal 750. The base station will respond by adjusting the weightingof the adaptive beamforming processor to direct a narrow beam D_(z) inthe desired direction.

One searcher beam S_(scan) is shown in FIG. 7. This beam can scan a full360 degrees, or depending on the local topography and objectives of thecellular system provider, the beam can scan only a sector thereof (suchas one of the sectors defined by lines 720, 730 and 740). The weightingof the searcher beam can additionally be changed as it scans the cell710 in the direction 770. The weighting could take into account anyinterference anomalies within the cell. For instance, the level ofinterference may be higher near the sector boundary denoted by line 730,and thus a stronger search beacon may be appropriate. As mentioned, morethan one scanning searcher beam can be used to interrogate the region.

The above described functions can be implemented through the exemplarycircuitry shown in FIG. 8. In this Figure, antenna elements 505, 510,515, selector unit 530, receiver 535, M-to-N_(s), selector 540 andsearcher circuit 545 are substantially similar to the like-numberedmodules shown in FIG. 5; thus, a detailed description thereof isomitted. FIG. 8 differs from FIG. 5 by replacing the fixed-beamprocessor 520 with the adaptive beamformer 800, and by including thebeamform controller 810 which controls the adaptive beamformer 800.

The adaptive beamformer 800 can comprise any conventional phased arrayadaptive beamformer controller, such as, but not limited to, theexemplary adaptive phased array beamformer shown in FIG. 2 of thepresent application. The adaptive beamformer unit 800, for instance cancomprise a plurality of weighting modules which apply weighting toindividual RF links. The controller 810 is used to control the weightingapplied by the adaptive beamformer 800 on the basis of feedbackinformation received from the output of the beamformer 800. Thecontroller 810 also receives a searcher control signal 555 from thereceiver 535, and a selector control signal 560 from the searchercircuit 545. Among other control information, these signals inform thebeamform controller of the members within the decoder set of beams sothat it can adjust its antenna weighting accordingly.

C. Other Applications

The structure and techniques disclosed above are not limited to theconventional cellular radio communication environment. These techniquescan be applied to other wireless applications, such as the indoorpicocell radio communication environment, or to various satellitecommunication environments.

For instance, FIG. 9 depicts a radio system architecture which providesindoor radio communication. As shown, a central hub station 910 iscoupled, through multiple high speed data transports 960, to a number ofdistributed radio head transceivers 915-950. The hub station 910 is alsocoupled, through an industry standard T1 TDM channel, to a mobileswitching center (MSC) 900. The MSC 900 is used to connect the localradio system comprising the hub 910 and the radio heads 915-950 to othercommunication networks (e.g., the public switched telephone network).Thus, mobile users within the coverage area of the radio heads 915 to950 (e.g. mobile user using terminal 905) can communicate with otherlocal users in the same coverage area or with remote users in othernetworks. The hub station 910 and the radio heads 915-950 might be used,for example, to provide mobile telephone and pager service within anoffice building, airport, large auditorium, or manufacturing plant.

FIG. 10 depicts an exemplary radio head (e.g. 950) which can be used toimplement the system of FIG. 9. As shown, an RF antenna 1040 isconnected to a duplexer 1050 which is in turn connected to a receive, oruplink, signal processing path and a transmit, or downlink, signalprocessing path. In the uplink signal processing path, the duplexer 1050is coupled to a low-noise amplifier (LNA) 1030 which is in turn coupledto an uplink heterodyne mixer 1000. The uplink mixer 1000 is connectedto an amplifier 1010 which is in turn connected to an analog-to-digitalconverter (ADC) 1020. The ADC 1020 feeds a parallel-to-serial dataconverter 1070 which in turn feeds an input of a high speed datatransport 1080 connected to the hub station (not shown). In the downlinksignal processing path, an output of the high speed data transport 1080is coupled to a serial-to-parallel data converter 1075 which is in turncoupled to a digital-to-analog converter (DAC) 1065. The DAC 1065 feedsa downlink heterodyne mixer 1060 which feeds a multi-carrier poweramplifier (MCPA) 1055. An output of the MCPA 1055 is connected to aninput of the duplexer 1050. Those interested in further detailsregarding indoor cellular communication systems are referred to commonlyassigned U.S. application Ser. No. 08/753,437, which is incorporatedherein in its entirety by reference.

On the receive path, the signals collected by the high speed transportline 1080 are transferred to the hub station 910 (in FIG. 9). The hubstation 910 processes the signals received from each radio head 915-950.In exemplary embodiments, the hub station 910 uses the same processingcircuitry shown in FIG. 5 to extract wanted signals from unwantedsignals received from the plurality of radio heads 915-950. In thiscase, however, instead of a phased array antenna having elements 505,510, and 515 controlled by a fixed beam former 520, the circuit shown inFIG. 5 would receive input from the links 960 connected to theindividual radio heads 915-950.

In the indoor context, the radio heads comprise a first set of “decoderradio heads”. The decoder radio heads are used for processing a callfrom one or more mobile terminals located within the vicinity of thedecoder radio heads. For instance, in the example of FIG. 9, radio heads945 and 950 may serve as decoder radio heads for communication withmobile terminal 905 located within a particular portion of a facility.

The signals received from the decoder radio heads are processed by acoherent demodulator, represented by receiver 535 in FIG. 5. Accordingto preferred embodiments, an interference-rejection-combining receiverdisclosed in commonly assigned U.S. application Ser. No. 07/284,775,filed on Feb. 8, 1994, is used to process the signals provided by thedecoder beams. This receiver combines signals received from each subsetof decoder beams and separates the wanted signals from the unwanted(interfering) signals.

The remainder of the radio heads shown in FIG. 9 which are not allocatedto decoding signals from mobile terminals are referred to as “searcherradio heads”. As in the case of searcher beams, the hub station 910 usesthe searcher radio heads to select candidates which should be added tothe currently active set of decoder radio heads by successivelyexamining the output of each searcher radio head. Those searcher radioheads (or combination of searcher radio heads) which meet prescribedcriteria are selected and allocated to the task of processing a callfrom a mobile terminal. In a manner similar to that discussed in SectionA of this patent, the hub station can detect the introduction andremoval of mobile terminals from the indoor environment, and can alsoconserve processing resources by reducing the duty cycle of the searcherradio head processing.

FIG. 11 shows the use of a base station 1100 which generates a pluralityof beams for communication with one or more orbiting satellites (e.g.satellite 1130 and 1140) travelling along trajectories (e.g. trajectory1150). The base station 1100 can employ the same circuitry shown in FIG.5 or 8 to locate satellites within communication range, and also toselect an optimal set of beams to communicate with the satellites oncethey have been detected.

Once again, the set of beams includes a first set of decoder beams (e.g.D_(W1) and D_(W2)) which are used to receive and decode signals from oneor more satellites (such as satellite 1130). The remainder of the beams(S₁-S₅) are used as searcher beams. The searcher beams are used toensure that the base station 1100 is using an optimal set of decoderbeams. FIG. 11 indicates that fixed beamforming is used, but a singlescanning searcher beam can be used in conjunction with an adaptivebeamforming processor (as in the embodiment of FIGS. 7 and 8). In otherrespects, the selection and processing of decoder and searcher beams issimilar to the above-described embodiments and thus a detailed discussedthereof is omitted.

Searcher beam processing can also be performed on-board the satellite.In this embodiment, the satellite moves in prescribed trajectory andpropagates a plurality of spot beams each having a prescribed coveragearea. The spot beams move as the satellite advances in its trajectory.The set of spot beams includes a first set of decoder beams which areused to receive and decode signals from one or more ground terminals.The remainder of the beams are used as searcher beams. As before, thesearcher beams are generally used to assist in the selection of one ormore decoder beams for use in communicating with one or more groundterminals using IRC, or some other technique, to remove interferencefrom the received signals. More particularly, the searcher beams assistin the identification of new and dropped calls, and assist in hand-overas a terminal moves from one beam coverage area to another.

Some satellites already employ conventional tracking systems which allowthese satellites to determine the precise location of ground terminals.Searcher beam processing can be used in such satellites to eithersupplement this conventional tracking capability (e.g., by performing aredundancy check), or can simply alleviate some of the demands placed onthe conventional tracking system (e.g., by relaxing the trackingprecision requirements imposed on the conventional tracking system).

In the on-board satellite embodiment, fixed or adaptive beamforming canbe used. The selection and processing of decoder and searcher beams issimilar to the above-described embodiments (e.g., as depicted in FIGS. 5and 8) and thus a detailed discussed thereof is omitted.

In yet another application, the searcher beams in any of the aboveembodiments can also carry paging messages in the downlink direction.The searcher beams can thus serve the joint role of searching for mobileterminals which wish to initiate a call, and also transmitting pagingmessages to any addressed pagers within a search area. In exemplaryembodiments, the searcher beam(s) only periodically visits each regionof the communication cell, e.g., at a reduced duty cycle. However, sincepaging messages can typically be communicated in short periodic bursts,the periodic nature of the searcher beam processing does not degrade theperformance of the downlink transmission of paging messages.

In the case of pagers which lack transmitting ability, the searcherbeams do not, properly speaking, “locate” the pagers, since the pagershave no ability to directly acknowledge reception of the searcher beams.The searcher beams simply broadcast information across a coverage areawhich may contain one or more addressed pagers. In the case of pagerswhich possess transmitting ability, however, the searcher beams canfunction in much the same manner as in the mobile radio cellularembodiment described above, and the searcher beams can be effectivelyused to locate the pagers and thereafter allocate one or more decoderbeams thereto.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. Thus the present invention is capable of many variations indetailed implementation that can be derived from the descriptioncontained herein by a person skilled in the art. All such variations andmodifications are considered to be within the scope and spirit of thepresent invention as defined by the following claims.

What is claimed is:
 1. A method for transmitting and receiving signalsin a radiocommunication system having a base station and at least onemobile station, the method comprising: generating a plurality of beamsfrom the base station including a first set of one or more of theplurality of beams for processing information from the at least onemobile station; searching for additional mobile stations by successivelyexamining a second set of one or more of the plurality of beams againstpredetermined criteria, comprising determining a rate at which thesecond set of beams is successively examined based on the speed at whichthe at least one mobile station is moving; and allocating the one ormore candidate beams to the first set if the predetermined criteria ismet.
 2. The method of claim 1, wherein successively examining a secondset of one or more of the plurality of beams against predeterminedcriteria further comprises determining the rate at which the mobilestation is moving based on Doppler shift of a signal transmissionemanating from the mobile station.
 3. A method for performing beamsearching using an adaptive beamforming array in a radiocommunicationsystem having a base station and at least one mobile station,comprising: generating a plurality of beams from the base stationincluding a first set of one or more of the plurality of beams forprocessing information from the at least one mobile station; directingthe first set of one or more of the plurality of beams toward the mobilestation so as to maximize a signal-to-interference ratio of a signalreceived from the mobile station; directing a single search beam over arange of orientations in search of an additional mobile station;examining the single search beam at each orientation in the range oforientations against a predetermined criteria; and allocating one ormore beams other than the search beam to the first set, the one or moreallocated beams corresponding to one or more orientations wherein thesingle search beam meets the predetermined criteria.
 4. The method ofclaim 3, wherein the range of orientations includes 360 degrees.
 5. Themethod of claim 3, wherein the range of orientations is determined bytopographical conditions in a coverage area.
 6. The method of claim 3,further comprising: applying a signal weighting to the single searchbeam; and changing the signal weighting over the range of orientations.7. The method of claim 3, wherein directing a single search beam overthe range of orientations comprises directing two or more search beamsover the range of orientations, and wherein examining the single searchbeam at each orientation comprises examining the two or more searchbeams at each orientation.
 8. The method of claim 7, wherein examiningthe two or more search beams at each orientation further comprises:transmitting a selection signal to a M-to-N selector; and analyzing N ofM of the two or more search beams according to the predeterminedcriteria.
 9. The method of claim 7, wherein the two or more search beamsinclude one or more paging messages in a downlink direction.
 10. Themethod of claim 3, wherein the radiocommunication system includes apicocell radiocommunication system.
 11. The method of claim 3, whereinthe radiocommunication system includes a satellite radiocommunicationsystem and the mobile station includes a satellite.
 12. The method ofclaim 3, wherein the single search beam includes one or more pagingmessages in a downlink direction.